First, the outage chart and tables were likely needed in a significant number of cases. The ICC regs on this, which date to 1918 at least, pertained to all commodities classified as inflammables. The 1923 revisions, which appear to be unchanged in the 1938 and 1949 updates (published in the CFR), include several common commodities. For example, both gasoline and naptha, when loaded at temperatures up to 65 F, required expansion volumes of 2.4 to 2.8%. At 75 F and above, the 2% minimum dome size would have been adequate. When loaded at 55 F, both ethanol and toluene would have required a ~2.8% expansion volume, while methanol and benzene would have needed about 3.2%. Commodities not classified as inflammable, including kerosene, diesel, fuel oil, etc., could all be carried with a 2% expansion space.
Second, tank cars actually built to the 2% minimum may have been in the minority. Using the Sinclair fleet of ca 1930 as an example, the GATC 1917 design cars had dome volumes of 2.5% (8000 gal) or 2.3% (10,000gal). The radial-course Penn Tank Car builds (8000 gal) had 3.2% domes. The notable exception seems to have been the 10,000-gal ACF Type 21s with their minimalist 210-gal domes, apparently the most common configuration for these cars. In contrast, the 8000-gal Type 21s most commonly sported 254-gal domes, and the P2000/Walthers Proto models of the Type 21s reflect these norms.
Last, starting some time in the early 1920s (perhaps 1923), the dome volumes of cars fitted with "side dome safety valves" were downgraded for all inflammable commodities. For example, Sinclair had 6000-gal ACF cars built in in 1918 to Class III the standard but with side-mounted valves. The 145-gal dome volume was downsized to only 95 gallons (~1.5%) for use with inflammables, so the outage table would have been a necessity for this combination of commodity and car. There are a number of other examples of cars so footnoted scattered throughout the 1936 and 1955 tariff books.